Both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) are well-known in the art. MWNTs generally comprise concentric cylinders with a hollow center and are capped on each end. The cylinders have inner diameters in the range of 2-10 nm and outer diameters in the range of 15-50 nm, and the spacing between each cylinder is similar to the inter-planar spacing of graphite. SWNTs generally comprise single layer tubes or cylinders in which a single layer of carbon is arranged in the form of a linear fullerene. The single layer tubes or cylinders comprising SWNTs generally have diameters in the range of about 1-2 nm. Both MWNTs and SWNTS have lengths on the order of microns, thus making them “high aspect ratio” particles.
Carbon MWNTs and SWNTs have a variety of unique electronic, optical, and mechanical properties that make them promising candidates for a wide range of applications, including, gas storage and separation, fuel cell membranes, batteries, photovoltaic devices, composite materials, electron emission materials for cold-cathode flat panel displays, and nanoscale wires and interconnects, just to name a few. However, before any of these applications can be effectively realized, a process must be developed for producing substantially defect-free and high purity carbon nanotubes quickly and on a large scale.
While several different methods for producing carbon MWNTs and SWNTs have been developed and are being used, none has provided an acceptable balance of high efficiency and low cost while producing substantial quantities of a highly pure, or at least a purifiable, MWNT or SWNT product. For example, arc discharge processes, while generally capable of producing modest quantities of nanotubes, also tend to produce excessive amounts of graphite and graphite encapsulated metals which are difficult to remove without destroying the product as well. Chemical vapor deposition (CVD) processes may also be used to produce modest quantities of nanotubes, but also tend to produce extraneous compounds which must be removed or separated in order to produce a purified product. Additionally, it is generally necessary to remove contamination from the catalyst support material, typically alumina or silica. Generally nanotubes produced by CVD processes are highly defective and therefore very difficult to purify. Laser vaporization methods are also known and have been developed to the point where they can produce relatively high yields of pure or easy to purify SWNTs. However, laser vaporization processes are very expensive and have not proven to be readily scalable to produce larger quantities of SWNTs.
Consequently, a need remains for methods and apparatus for producing MWNTs and SWNTs capable of producing a relatively pure, or at least an easy to purify, product at a relatively low cost. Additional advantages would be realized if such processes were continuous and readily scalable, thereby allowing for the large scale, economical production of a highly pure MWNT and SWNT product.